The field of electromagnetic wave propagation is a well-developed branch of physics and various physical models have been developed and supported by appropriate mathematical formulations, which includes the propagation of optical field in waveguides including optical fibers. With the advent of a phenomenon called morphology dependent resonances or whispering gallery modes in dielectric resonant cavities like spheres and cylinders, optical waveguides have been used to deliver the optical field to cavities. The resonances observed and reported have been, in most cases, explained only in terms of properties of the resonant cavities and the environment. Effects of the means of the light delivery, however, have never been discussed.
A dielectric microsphere (or similar geometry is an optical structure that exhibits resonant properties, which means that the microsphere can select very narrow segments of an incoming signal's spectrum for further manipulation and processing. As mentioned above, the optical resonances of a microsphere are frequently called the whispering gallery modes (WGMs). In general, microspheres belong to the same group of devices as Fabry-Perot interferometers and fiber Bragg gratings.
The optical resonances in microspheres are a function of their morphology (i.e., geometry and dielectric properties (refractive index)). Any perturbation to their morphology caused by a change in the surrounding environment will lead to a shift in the resonances (WGM). By tracking these morphology-dependent shifts of WGMs, it is possible to measure the change in a given environmental property. In comparison to Fabry-Perot interferometers and fiber Bragg gratings, morphology-dependent-resonance (MDR) microsphere sensors can be built in smaller sizes. The small size permits the use of these microspheres in a very dense fashion on a very small footprint, so compact multipurpose devices for sensing could be constructed.
In addition, microsphere tends to exhibit significantly higher quality factors Q, which is defined as Q=Δ/λΔ, where Δ is the wavelength at which a resonance occurs and λΔ, is the line width of the resonant wavelength. The high Q-value offers the potential of a very high sensitivity of the measured quantity. Devices that employ high Q-value MDRs have been reported relating to applications in communications and biological sensors. In communication, MDR-based channel dropping filters and modulators have been demonstrated. Further, using the recent advances in microlithography, several MDR-based devices have been made on a common substrate permitting coupling signal at resonant modes from one resonator to another. In these applications, the resonator is perturbed externally to affect a change in its morphology.
In sensor applications, the MDR device is a passive element where the change in a specific environmental condition perturbs the morphology of the resonator, which is measured quantitatively by monitoring the resonance shifts. In the case of biological sensing, the optical resonance shifts occurs due to changes in the physical conditions of the surrounding medium alone (without inducing a perturbation to the morphology of the resonator itself). For example, a change in the refractive index of the surrounding medium alone will have an effect on the structure of sphere resonances and may be used to detect the presence of a certain chemical or a biological agent.